1. Field of the Invention
The invention relates to methods for fabricating integrated optical isolators. More specifically, the invention relates to a method for forming a wire grid polarizer on a Faraday rotator and a method for suppressing reflection of rejected polarization.
2. Background Art
Fiber-optic communications systems have three major components. These include (1) a transmitter that converts electronic data signals to light signals, (2) an optical fiber that guides the light signals, and (3) a receiver that captures the light signals at the other end of the fiber and converts them to electrical signals. For high-speed data transmission or long-distance applications, the light source in the transmitter is usually a semiconductor laser diode. The transmitter pulses the output of the laser diode in accordance with the data signal to be transmitted and sends the pulsed light into the fiber. In fiber-optic communications systems, some light may be reflected back from the fiber network. This back reflection affects the operation of the laser diode by interfering with and altering the frequency of the laser output oscillations. For this reason, an optical isolator is typically provided between the laser diode and the optical fiber to minimize the back reflection from the fiber network.
FIG. 1 shows a prior art optical isolator 2 which comprises a magneto-optical material 4, called a Faraday rotator, sandwiched between an entrance polarizer 6 and an exit analyzer polarizer 8. The polarizers 6, 8 are typically polarizing glass chips. The exit analyzer polarizer 8 is set at 45xc2x0 relative to the entrance polarizer 6. The Faraday Rotator 4 and the polarizers 6, 8 are surrounded by a permanent magnet 10, which applies a magnetic field to the Faraday rotator 4. The magnetic field in concert with the Faraday rotator 4 causes the plane of polarization of the incident beam 12 to rotate 45xc2x0 within the Faraday rotator 4, thus allowing the incident beam 12 to pass through the exit analyzer polarizer 8. The transmitted beam is indicated at 14. Any reflected light that travels in the reverse direction is first polarized at 45xc2x0 by the exit analyzer polarizer 8. The Faraday effect is non-reciprocal. Thus the light that passes through the Faraday rotator 4 is rotated an additional 45xc2x0 and is then blocked by the polarizer 6.
To ensure desired characteristics of the optical isolator 2, the polarizers 6, 8 must be precisely aligned with Faraday rotator 4 so that the appropriate angle is formed between the polarizers 6, 8. Because of the alignment requirements, the assembly process of the optical isolator 2 is somewhat labor-intensive. Some manufacturers use manual methods for assembly followed by soldering, gluing, or welding techniques to fix the individual components in place. The materials used to fix the components in place present reliability problems in terms of micro movement of the components in hostile operating conditions. U.S. Pat. No. 5,757,538 issued to Siroki proposes a solution which includes forming polarizing wire grids on both surfaces of a Faraday rotator. The wire grids on the surfaces of the Faraday rotator are used in lieu of the polarizing glass chips 6, 8. The proposal of forming wire grid polarizers on the Faraday rotator suggests that a non-manual/automated process is envisioned. However, the Siroki patent does not indicate how this is done.
Several techniques are available for forming wire grid structures on substrates. One technique known as photolithography involves transferring a wire grid pattern on a photomask to a surface of the substrate. A metal is first deposited on the substrate. Then a photosensitive material, called a photoresist, is applied on the metal layer. The photomask with the wire grid pattern is aligned with the substrate so that the pattern can be transferred to the photoresist. Once the photomask is aligned with the substrate, the photoresist is exposed through the pattern on the photomask with a high intensity ultraviolet light. Mask patterning in the photoresist could be made, for example, by exposing the photoresist to an interference pattern induced by a He-Cd laser. See M. Koeda et al., xe2x80x9cProduction of Metallic Grating,xe2x80x9d PAJ-06174907, Jun. 24, 1994. The pattern formed in the photoresist is transferred to the metal layer by etching, e.g., reactive ion etching or ion milling.
Photolithography is widely used in fabricating wire grid polarizers that operate in the mid infrared region (approximately 3 xcexcm to 25 xcexcm). However, photolithography has limited application in the near infrared region (approximately 0.75 xcexcm to 3 xcexcm) because the grid structure in this region requires submicron features. Currently, the smallest line width that can be made with photolithography is approximately 0.2 xcexcm. The commercial technology for fabricating submicron patterns is electron beam lithography (xe2x80x9cEBLxe2x80x9d). EBL involves scanning a beam of electrons across a surface covered with a resist film that is sensitive to those electrons. Other methods for fabricating wire grid polarizers are disclosed in S. Kawakami and H. Tsuchiya, xe2x80x9cPolarizing Element,xe2x80x9d PGJ-61-16991, May 2, 1986, T. Katsuragawa et al, xe2x80x9cPolarizer, Its Production and Display or Display Device Provided with Polarizer,xe2x80x9d PAJ-10213785, Aug. 11, 1998, and Y. Sato, xe2x80x9cProduction of Grid Type Polarizer,xe2x80x9d PAJ-09090122, Apr. 4, 1997.
EBL is capable of forming fine patterns but is much slower and generally costlier than photolithography. U.S. Pat. No. 5,772,905 issued to Chou discloses a cost-effective process for forming submicron features on a substrate. The process, called nano-imprint lithography, is essentially an embossing technology. In nano-imprint lithography, a pattern is formed on a mold by such methods as EBL and subsequent etching processes. A mold made in such fashion is used in the embossing process. The mold is brought into contact with a thin film carried on a surface of a substrate, e.g., a silicon wafer, so that the pattern on the mold can be embossed on the thin film. The thin film layer comprises a thermoplastic polymer, e.g., polymethyl methacrylate (PMMA). During the embossing step, the thin film, the substrate, and the mold are heated to allow sufficient softening of the polymer. At this time, pressure is applied to emboss the polymer. After a period of time, the entire assembly is cooled below the glass transition temperature of the polymer and the mold.
Two options are available after embossing the polymer depending on whether the polymer film is carried directly on the surface of the substrate or on a metal layer deposited on the surface of the substrate. If the polymer film is carried directly on the surface of the substrate, the thin sections of the embossed polymer are removed, e.g., by oxygen etching, to expose the underlying substrate. After removing the thin sections, a metallic material is deposited on the exposed substrate. Then, a lift-off process is used to remove the remaining polymer from the substrate. If the polymer film is carried on a metal layer deposited on the surface of the substrate, an etching process such as reactive ion etching or ion milling is used to etch into the metal layer. While etching into the metal layer, the polymer on the metal layer is also removed. If the polymer is not completely removed from the metal layer by the time the pattern is etched into the metal layer, a solvent may be used to remove the remaining polymer.
Before fabricating the mold used in the embossing process, the wire grid pattern on the mold is modeled to ensure that the performance of the resulting wire grid polarizer is acceptable. There are several mathematical models and expressions that can be used to determine the performance of the wire grid polarizer with respect to transmission of parallel and perpendicular electric fields of light. These mathematical models could be based, for example, on Maxwell""s theory, transmission line theory, rigorous coupled wave analysis (xe2x80x9cRCWAxe2x80x9d), etc. Conventional modeling of wire grid polarizers using these methods are generally metallic lines on transparent substrates which would produce useful polarizer. FIG. 2 shows an example of such a wire grid polarizer with parallel metallic lines 16 on a transparent substrate 18. In operation, the polarized light that is not transmitted through the substrate 18 is reflected off the metallic surfaces 16.
In one aspect, the invention relates to a method for fabricating an integrated optical isolator which comprises depositing a wire grid material on a magneto-optical substrate and depositing a resist film on the wire grid material. The method further includes bringing a mold with a wire grid pattern in contact with the resist film and compressing the mold and resist film together so as to emboss the wire grid pattern in the resist film. The method further includes transferring the wire grid pattern in the resist film to the wire grid material on the magneto-optical substrate by etching.
In some embodiments, the wire grid material comprises a metallic material. In some embodiments, the wire grid material comprises a dielectric material sandwiched between two metallic materials. In one embodiment, the metallic materials are selected from the group consisting of Al, Au, Cu, Ir, Mo, Ni, Os, Pt, Rh, and W. In one embodiment, the dielectric material is selected from the group consisting of Si, SiO2, and GaAs.
Other aspects and advantages of the invention will be apparent from the following description and the appended claims.